For detecting changes in the electrical field under water, pairs of different kinds of electrodes, made of zinc, graphite, silver/silver chloride etc., have previously been used. They have all had their benefits and drawbacks, such as silver/silver chloride electrodes having high sensitivity but at high costs, zinc electrodes having a low price but being mechanically or electrically instable, and graphite electrodes which are difficult to manufacture according to defined specifications without discarding. Large active areas have required large physical dimensions and have, in connection with graphite electrodes, caused irreproducible properties conditioned by small chemical or morphological variations. Consequently, there are significant problems with the previously available techniques.